Improve pull mirroring error message

Problem

The customer encountered the following error during the pull mirroring process.

{
  "id": **,
  "update_status": "failed",
  "url": "https://*****:*****@gitlab.com/**/**/**.git",
  "last_error": "9:Could not update main. Please refresh and try again..",
  "last_update_at": "2025-01-29T13:53:06.360Z",
  "last_update_started_at": "2025-01-29T13:53:05.218Z",
  "last_successful_update_at": "2025-01-29T11:07:06.717Z",
  "enabled": true,
  "mirror_trigger_builds": true,
  "only_mirror_protected_branches": true,
  "mirror_overwrites_diverged_branches": true,
  "mirror_branch_regex": null
}

The root cause was identified: the mirror user didn't have force push permissions on the branch to allow to overwrite divergence.

Proposal

1. Try to reproduce this problem based on the report
2. Consider improving the error message to make it clearer.